The present invention relates to a valve assembly having a diffuser flow passage and, in particular, a hermetically sealed valve having a high efficiency diffuser flow passage wherein the flow therethrough is controlled by a diaphragm actuated spring biased closure member lying substantially entirely upstream of the passage to thereby be isolated from valve pull down forces during high flow conditions in the passage.
A valve assembly having an improved flow passage for a diaphragm or bellows type isolation valve is disclosed in the U.S. Pat. application Ser. No. 402,165 filed in the name of E.B. Pool et al on Oct. 1, 1973 and assigned to the assignee of the present invention. Therein, axial movement of the valve closure member is limited by the permissible flexure of a metal diaphragm disc which hermetically separates the valve stem from the former and the valve control chamber. To prevent an excessive pressure drop across the valve seat such as would normally occur due to the thus limited movement, a high efficiency annular diffuser passage is provided between a frusto-conical surface on the valve closure member and opposed frusto-conical surface downstream on the valve seat. The diffuser passage prevents the high velocity fluid from expanding too rapidly and thus dissipating the fluid head.
While providing a distinct advantage over prior low lift valve designs, we have determined that under certain high flow conditions, a hydrodynamic pull down force was exerted on the valve closure member due to the low pressure conditions in the diffuser passage. Depending on the spring load biasing closure member toward disphragm and the flow velocity through the passage, the resultant force on the closure member can cause the latter to move to a partially opened equilibrium position. This can occur with a valve initially in a stable fully open position if the flow is increased to exceed a threshold level where the pulldown force exceeds the spring force, or it can occur during the valve opening cycle or the valve closing cycle.
During the valve opening cycle, the pull down force can prevent the valve from opening beyond the equilibrium position regardless of valve stem position. During the valve closing cycle, as soon as threshold conditions are reached, the valve can immediately retract to the equilibrium position. While this effect can be partically obviated by increasing the valve diameter, this cannot be totally corrected even where sufficient valve seat space is available to permit enlargement thereof.
The present invention eliminates the aforementioned valve pull down forces by incorporating a fixed diffuser assembly which substantially bears all pull down forces developed in the diffuser passage. The diffuser assembly has a frusto-conical diffuser surface which defines with an opposed surface downstream of the valve seat the optimum flow passage conditions for a given valve design. The diffuser surface is permanently fixedly positioned at a location corresponding to the fully open position of the valve closure member. The diffuser assembly has a central axial bore in which a spring biased valve closure disc is slidably supported. The closure disc has an annular seating surface which is axially movable with respect to the opposed annular valve seat under the control of an axially movable valve steam hermetically separated from the closure disc by a flexible diaphagm. In the closed position, the closure disc completely closes the flow passage. In the open position, the aforementioned surfaces establish the diffuser passage. The fluid exposed surfaces of the closure disc are located inwardly of the diffuser passage and thus are not exposed to the pressure differentials of the prior assembly. This eliminates valve pull down forces regardless of valve seat diameter, permissible valve lift, flow velocity or spring load.
The above and other features of the present invention will be apparent to those skilled in the art upon reading the accompanying detailed description, reference being made to the accompanying drawings illustrating a preferred embodiment thereof.